1. Field of the Invention
The present invention relates to oxadiazole derivatives; light-emitting elements including the oxadiazole derivatives; and display devices, lighting devices and electronic devices each using the light-emitting element.
2. Description of the Related Art
In recent years, research and development of light-emitting elements using electroluminescence have been extensively conducted. In a basic structure of such a light-emitting element, a layer including a light-emitting substance is interposed between a pair of electrodes. By applying voltage to this element, light emission can be obtained from the light-emitting substance.
Since this type of light-emitting element is a self-luminous type, it has advantages over a liquid crystal display in that visibility of an image is high and that no backlight is needed, and is thought to be suitable as flat panel display elements. Further, such a light-emitting element also has advantages in that the element can be formed thin and lightweight and that response time is extremely high.
Further, since this type of a light-emitting element can be formed to have a film shape, surface light emission can be easily obtained by formation of large-area elements. This feature is difficult to realize with point light sources typified by a filament lamp and an LED or with linear light sources typified by a fluorescent light. Such light-emitting elements therefore have a high utility value as a surface light source that can be applied to lighting devices or the like.
Light-emitting elements using electroluminescence are broadly classified depending on whether they use an organic compound or an inorganic compound as a light-emitting substance. When an organic compound is used as a light-emitting substance, by application of voltage to a light-emitting element, electrons and holes are injected from a pair of electrodes into a layer including the light-emitting organic compound, and current flows. Carriers (i.e., electrons and holes) then recombine to excite the light-emitting organic compound. The light-emitting organic compound relaxes to a ground state from the excited state, emitting light.
The light-emitting element which works on this principle is called a current-excitation light-emitting element. Note that an excited state of an organic compound can be of two types: a singlet excited state and a triplet excited state. In addition, luminescence from the singlet excited state (S*) is referred to as fluorescence, and luminescence from the triplet excited state (T*) is referred to as phosphorescence. Furthermore, it is thought that the ratio of S* to T* in a light-emitting element is statistically 1:3.
At room temperature, a compound that converts a singlet excited state into luminescence (hereinafter referred to as a fluorescent compound) exhibits only luminescence from the singlet excited state (fluorescence), not luminescence from a triplet excited state (phosphorescence). On the basis that S*:T*=1:3, the internal quantum efficiency (ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent compound is thought to have a theoretical limit of 25%.
However, with use of a compound that converts a triplet excited state into luminescence (hereinafter referred to as a phosphorescent compound), the internal quantum efficiency can theoretically be 75% to 100%. That is, the emission efficiency can be three to four times as high as that of a fluorescent compound. From that reason, in order to achieve a light-emitting element with high efficiency, a light-emitting element using a phosphorescent compound has been actively developed in recent years (e.g., see Non-Patent Document 1).
When the above-described phosphorescent compound is used in a light-emitting layer of a light-emitting element, in order to suppress concentration quenching of the phosphorescent compound or quenching due to triplet-triplet annihilation, the light-emitting layer is often formed so that the phosphorescent compound is dispersed in a matrix including another substance. In that case, a substance serving as a matrix is referred to as a host material, and a substance that is dispersed in a matrix, like a phosphorescent compound, is referred to as a guest material.
When a phosphorescent compound is used as a guest material, a host material is needed to have triplet excitation energy (an energy difference between a ground state and a triplet excited state) higher than the phosphorescent compound. It is known that CBP which is used as a host material in Non-Patent Document 1 has higher triplet excitation energy than a phosphorescent compound which exhibits emission of green to red light, and is widely used as a host material of the phosphorescent compound.
Despite the high triplet excitation energy, CBP is poor in ability to receive holes or electrons, which results in a problem in that driving voltage gets higher. In view of the above problem, a substance that has high triplet excitation energy and also can easily accept or transport both holes and electrons (i.e. a bipolar substance) is required as a host material of a phosphorescent compound.
Because singlet excitation energy (an energy difference between a ground state and a singlet excited state) is greater than triplet excitation energy, a material that has high triplet excitation energy will also have high singlet excitation energy. A bipolar substance having high triplet excitation energy is therefore also useful as a host material in a light-emitting element formed using a fluorescent compound as a light-emitting substance.